The relative value of call embellishment in túngara frogs

Abstract

Facultative traits that have evolved under sexual selection, such as the acoustic ornaments present in the advertisement signals of male túngara frogs (Physalaemus pustulosus), offer a unique opportunity to examine selection for trait exaggeration with a focus on individual differences amongst signalers. By contrast, many studies of mate choice use experimental designs that obscure the inter-individual variation amongst signalers available for selection to act on—through the use of “typical” or average signals from the population. Here, we use dichotomous female phonotaxis choice tests to determine how the value of male call embellishment varies across 20 individual males frogs recorded from the wild—a sample which captures the acoustic diversity present in the population. We tested 20 females for each male call pair (i.e., 400 females). The results show widespread preference amongst females for ornamented calls (“whine–chucks”) over simple calls (“whines”), yet also demonstrate substantial variation in the relative benefits for individual male frogs—some males enjoy appreciable benefits by using ornaments while others (30% of males in this study) do not. We also show that the relative amplitude of the chuck to the whine correlates positively with the value of call elaborations across these 20 males. Finally, by manipulating the relative amplitude of whines and chucks using both natural and synthetic calls, we demonstrate directly that this single call parameter is key to determining the relative value of call elaborations across males.

Notes

Acknowledgments

We thank X. Bernal, A. Lea, L. Ziegler, A. Dapper, and N. Lessios for assistance with data collection and the staff at the Smithsonian Tropical Research Institute for logistical support. The suggestions of two anonymous reviewers significantly improved this manuscript. Autoridad Nacional del Ambiente approved scientific permits in the Republic of Panamá. Animal use was approved by the IACUC (#06041701) at The University of Texas at Austin. This work was supported by the National Science Foundation (grant number IOB 0544096 to M. Ryan). The Homer Lindsay Bruce Endowed Fellowship supported A. Baugh.